Fuel cell module

ABSTRACT

A fuel cell module includes a number of series-connected fuel cells forming a fuel cell stack. To reliably ensure that there is no risk of the fuel cell stack buckling, even where a comparatively large number of fuel cells are grouped to form a fuel cell stack, the fuel cell stack is surrounded by a stabilizing casing, at least in a middle area as seen in the longitudinal direction.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/DE01/03542 which has an Internationalfiling date of Sep. 14, 2001, which designated the United States ofAmerica and which claims priority on German Patent Application number DE100 47 591.4 filed Sep. 26, 2000, the entire contents of which arehereby incorporated herein by reference.

FIELD OF THE INVENTION

The invention generally relates to a fuel cell module in which a numberof fuel cells connected in series form a fuel cell stack.

BACKGROUND OF THE INVENTION

Fuel cells can be used for the environmentally friendly generation ofelectricity. This is because a process which substantially represents areversal of electrolysis takes place in a fuel cell. For this purpose,in a fuel cell, a fuel which includes hydrogen is fed to an anode and anauxiliary substance which includes oxygen is fed to a cathode. The anodeand cathode are electrically separated from one another by anelectrolyte layer; although the electrolyte layer does allow ionexchange between the fuel and the oxygen, it otherwise ensures gas-typeseparation of fuel and auxiliary material.

On account of the ion exchange, hydrogen contained in the fuel can reactwith the oxygen to form water, during which process electrons accumulateat the fuel-side electrode, i.e. the anode, and electrons are depletedat the electrode on the auxiliary substance side, i.e. the cathode.Therefore, when the fuel cell is operating, a potential difference orvoltage is built up between the anode and cathode. The electrolytelayer, which in the case of a high-temperature fuel cell may be designedas a solid ceramic electrolyte or in the case of a low-temperature fuelcell may be designed as a polymer membrane, therefore has the functionof separating the reactants from one other, of transferring the chargein the form of ions and of preventing an electron short circuit.

On account of the electrochemical potentials of the substances which areusually used, in a fuel cell of this type, under normal operatingconditions, an electrode voltage of approximately 0.6 to 1.0 V can bebuilt up and maintained during operation. For technical applications, inwhich a significantly higher overall voltage may be required dependingon the intended use or the planned load. Therefore, it is usual for aplurality of fuel cells to be connected electrically in series, in theform of a fuel cell stack, in such a way that the sum of the electrodevoltages which are in each case supplied by the fuel cells correspondsto or exceeds the required total voltage.

Each of the fuel cells which are combined to form a fuel cell stack ofthis type is assigned, in the region of its electrode, a volume regionto which the media required in each case, such as for example the fuelor the auxiliary material, can be fed. This volume region may, forexample, be delimited by boundary surfaces which are counted as part ofthe fuel cell itself, the boundary surfaces between two adjacent fuelcells, in order to form a closed volume region, being sealed off fromthe outside by means of a seal arranged between them. Depending on thetotal voltage required, the number of fuel cells in a fuel cell stack ofthis type may, for example, be 50 or more.

To produce the required seal between adjacent fuel cells in terms of thesupply and discharge of the media, such as fuel and auxiliary substance,it may be necessary to subject the fuel cell stack to a certain clampingor pressing in its longitudinal direction. This corresponds to along-term compressive load acting on the fuel cell stack in itslongitudinal direction. This ensures that, firstly each fuel cellremains in mechanical contact with the fuel cells adjoining it, and,secondly, particularly when seals made from elastic material are usedbetween the fuel cells, the required sealing action is indeed achievedas a result of the pressure applied in the longitudinal direction.

However, with elongate structures, such as bars, towers or struts,according to the Euler or Tetmajer buckling conditions, such acompressive load may result in a tendency to buckle. In this context,the term buckling is to be understood in particular as meaning yieldingof a central region of the elongate structure in a directionperpendicular to the longitudinal axis. This tendency to buckle isdependent to a considerable extent on the length of the structure inquestion.

Buckling of this nature which occurs in a fuel cell stack, as a resultof some of the fuel cells being moved out of their desired position,would, however, have a highly adverse effect on or even completelynegate the ability of the fuel cell stack to function. The number offuel cells which can be connected to one another to form a fuel cellstack is therefore only limited, depending on the sealing system whichis provided for the connection of adjacent fuel cells and the resultingclamping force required in the longitudinal direction of the fuel cellstack.

On the other hand, however, particularly when a fuel cell system isdesigned for applications with relatively high design voltages, it ispossible to provide for a relatively high number of fuel cells, forexample 70 or more, to be connected up. The possibility of combining anydesired number of fuel cells to form a fuel cell stack thereforerepresents an important contribution to the flexibility available indesigning a fuel cell module. In particular for flexibility reasons, itmay be desirable for the fuel cells to be combined to form a fuel cellstack in a readily portable fuel cell module.

SUMMARY OF THE INVENTION

Therefore, an embodiment of the invention is based on an object ofproviding a fuel cell module in which, even when a relatively largenumber of fuels cells are combined to form a fuel cell stack, the riskof the fuel cell stack buckling is reliably avoided.

According to an embodiment of the invention, this object may be achievedby the fuel cell stack being surrounded by a stabilizing casing at leastin a central region, as seen in its longitudinal direction.

An embodiment of the invention is based on the consideration that theundesirable tendency of the fuel cell stack to buckle, although it is onthe one hand dependent on the number of the fuel cells which form thefuel cell stack, it is also, on the other hand, dependent on thecharacteristics of the mechanical contact between two adjacent fuelcells. For sufficient flexibility when designing a fuel cell module,however, the mechanical stability of the fuel cell stack should beprovided independently of these two parameters. For this purpose,external stabilization of the fuel cell stack is provided.

To make it particularly simple and therefore advantageous to assemble,the stabilizing casing advantageously has a number of elements which arematched in a positively locking manner to the outer contour of the fuelcell stack.

In a further advantageous configuration, some or all of the elements aredesigned as angle bars which can be placed against an outer edge of thefuel cell stack. In an advantageous alternative configuration, some orall of the elements are designed as a U-shaped metal sheet which isextended in the longitudinal direction of the fuel cell stack andengages around the latter in cross section on one of its outer sides. Acommon feature of both configurations of the elements is that in crosssection they have at least one angular region which is preferably ofright-angled design.

This on the one hand ensures a basic strength or rigidity of the elementwith respect to twisting or torsion and, on the other hand, ensures acertain stability with respect to lateral loads or buckling. Therefore,a contribution is made to stabilizing the fuel cell stack with respectto buckling not only by the interaction of a plurality of the elementsduring the formation of the stabilizing casing but also simply by anindividual element per se.

To form the stabilizing casing, in a further advantageous configurationeach element is connected to the elements which adjoin it by use of anumber of connecting elements. In this way, the elements can bedesigned, in a particularly material-saving form, as relatively narrowelements which, although they have the mechanical strength preventingbuckling, cover only a relatively small part of the actual surface ofthe fuel cell stack. In this case, they can be assembled to form thestabilizing casing by use of the connecting elements which, depending onthe particular design, only have to satisfy significantly lowermaterials requirements than the actual elements.

The connecting elements are expediently designed as tensioning struts ortensioning straps. With a design of this type, the connecting elements,in the assembled state, are substantially subject to tensile load, sothat particularly appropriate materials can be used.

When fuel cells are connected up to form a fuel cell stack,voltage-carrying components or electrodes may be accessible inparticular at the edge regions of the fuel cells. In order,nevertheless, to reliably rule out the risk of short circuits and aresultant failure of the fuel cell module even when it is used in thevicinity of exposed or grounded parts, each element is advantageouslyprovided with a number of insulating elements on its side which facesthe fuel cell stack. Therefore, depending on the design of thestabilizing casing, it may also be designed as an insulting sheath forthe fuel cell stack. In any event, however, the insulating elementsensure that the elements of the stabilizing casing themselves are keptfree from potential and therefore do not contribute to the risk of shortcircuits being formed.

The fuel cells which lie on the outer sides, as seen in the longitudinaldirection of the fuel cell stack, i.e. the first and last fuel cells asseen in the stack direction, are advantageously each connected to aconnection plate, which is also known as a terminal plate. Theconnection plates are preferably of metallic design and are used tosupply and remove operating media and the operating current to and fromthe fuel cell. The connection plates are of sheet-like design, in orderto ensure a correspondingly uniform introduction or removal of theoperating current into and from the active region of the first and lastfuel cells.

To make it possible to achieve a particularly mechanically stableconfiguration of the fuel cell module in a particularly compactarrangement, a number of tie rods are advantageously arranged on eachconnection plate. In a further advantageous configuration, theconnection plates are clamped together in the longitudinal direction ofthe fuel cell stack via these tie rods by use of clamping devicesengaging on the latter. In addition to producing a mechanically stableand therefore also readily portable unit, this makes it possible, in aparticularly simple way, to maintain a desired axial pressure on eachindividual fuel cell, in particular to maintain the requiredleaktightness of the seals in the abovementioned way.

The advantages which are achieved by at least one embodiment of theinvention reside, in particular, in the fact that the encasing of atleast a central region of the fuel cell stack with a stabilizing casingensures a particularly high stability with respect to lateral bucklingeven when components which are actually unsuitable in mechanical terms,such as for example the sealing elements in the fuel cell stack, areused. It is therefore possible to apply an axial pressure to the fuelcell stack in order to ensure a required leaktightness in the mediaspaces between the individual fuel cells without the operationalreliability of the fuel cell module being adversely affected. Theclamping of the fuel cell module in the axial direction, in particularthe use of the tensioning means in combination with the tie rodsarranged on the terminal plates, also makes it possible to provide aparticularly compact and relatively easily portable fuel cell module.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention is explained in more detailwith reference the drawings, in which:

FIG. 1 shows a side view of a fuel cell module, and

FIG. 2 shows a cross section through the fuel cell module as shown inFIG. 1.

Identical parts are provided with identical reference numerals in bothfigures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The fuel cell module 1 as shown in FIG. 1 includes a number of fuelcells 2, which are only indicated in the figure and are electricallyseries-connected. Each fuel cell 2, in a manner which is not illustratedin more detail, includes a sheet-like electrolyte which is covered overa large area on both sides by an electrode. The electrode has a numberof passages passing through it in each case, via which a medium, inparticular depending on the polarity of the corresponding electrode, afuel which includes hydrogen or an auxiliary substance which includesoxygen, can be brought into contact with the electrolyte. For theelectrical series connection, the fuel cells 2 are connected to oneanother in pairs by means of their electrodes, with the overall resultbeing the formation of a fuel cell stack 4 in the manner of a layeredstructure.

To achieve sufficient operational reliability when the fuel cells 2 areoperating, and in particular to produce a leaktightness, which issufficient for operational reliability, at the seals which are arrangedbetween each pair of adjacent fuel cells 2, it is intended for the fuelcell stack 4 to be operated under a certain axial pressure in itslongitudinal direction, which is indicated by the arrow 6. However, theapplication of an axial pressure of this type in the longitudinaldirection of the fuel cell stack 4 leads to the fuel cell stack 4 havinga tendency to buckle, in particular on account of the elastic materialswhich are provided for the seal between each pair of fuel cells 2. Inthis context, the term buckling is to be understood as meaning inparticular one or more fuel cells 2 shifting out of the central regionof the fuel cell stack 4 in a direction perpendicular to thelongitudinal direction represented by the arrow 6.

The tendency of the fuel cell stack 4 to buckle also increases, interalia, as the number of fuel cells 2 connected in series increases.Therefore, the number of fuel cells 2 which can be connected in seriesto form the fuel cell stack 4 is essentially limited by the definitionof a buckling tendency on the part of the fuel cell stack 4 which canstill be considered tolerable for sufficient operational reliability.

To avoid this undesirable restriction in the flexibility of the fuelcell module 1 and to make it possible for any desired number of fuelcells 2 to be connected in series, in particular according to thecorresponding design load, irrespective of the materials used whendesigning the fuel cells 2, the fuel cell module 1 is designed for aparticularly high stability with respect to the abovementioned risk oflateral buckling. For this purpose, the fuel cell stack 4 is surroundedby a stabilizing casing 10 over a relatively large longitudinal region8, as seen in its longitudinal direction represented by the arrow 6. Thestabilizing casing 10 comprises a number of elements 12 which arematched in a positively locking manner to the outer contour of the fuelcell stack 4.

In the exemplary embodiment, the elements 12 are each designed as anglebars which can be placed against an outer edge of the fuel cell stack 4.Alternatively, it is also possible to provide elements which aredesigned as U-shaped metal sheets which are extended in the longitudinaldirection of the fuel cell stack 4 and engage around the latter in crosssection on one of its outer sides.

Each element 12 is connected to the elements 12 which adjoin it in eachcase by way of a number of connecting elements 14, which in theexemplary embodiment are designed as tensioning struts. Some of theconnecting elements 14 are designed as pairs of connecting elements 14which cross one another diagonally. On the other hand, other connectingelements 14 are designed as transverse struts which run on their own andare oriented substantially perpendicular to the longitudinal directionof the fuel cell stack 4 as indicated by the arrow 6.

The fuel cells 2 which lie on the outer sides, as seen in thelongitudinal direction of the fuel cell stack 4, are each connected to aconnection plate 16, 18.

The operating media can be fed to the fuel cells 2 which are connectedup to form the fuel cell stack 4 via the connection plates 16, 18.

A number of tie rods 20 are formed integrally on each of the connectionplates 16, 18 which are also known as terminal plates. The connectionplates 16, 18 are clamped together, as seen in the longitudinaldirection of the fuel cell stack 4, by way of the tie rods 20 andtensioning straps or tensioning devices which are attached to the tierods and are not shown in more detail. This clamping leads to a pressurebeing exerted on the fuel cells 2 and the sealing elements fittedbetween them in the axial direction or longitudinal direction of thefuel cell stack 4. Therefore, this arrangement makes the fuel cellmodule 1 particularly compact, so that it can be used as a portableunit.

FIG. 2 shows the fuel cell module 1 in cross section. The figure shows aplan view of two adjacent fuel cells of the fuel cells 2 which form thefuel cell stack 4. Each of the fuel cells 2 has an electrode 21 which isof sheet-like design and can be brought into contact with acorresponding electrode of a fuel cell 2 which adjoins it in the stackdirection, so that the fuel cells 2 are connected in series. As can beseen from the cross section shown in FIG. 2, the fuel cell 2 andtherefore the fuel cell stack 4 which it forms is surrounded by theelements 12 in the edge regions. A number of insulating elements 22 arein each case arranged between the elements 12 and the fuel cell 2. Theinsulating elements 22 ensure that the actual potential-carrying fuelcells 2 are in electrical terms completely decoupled from the elements12. Consequently, the elements 12 can be grounded, so that the fuel cellmodule 1 has a particularly high operational reliability.

Between the two fuel cell stacks 4 there are electrically insulatingspacers 24 which hold the two stacks at a predetermined distance fromone another. The spacers 24 are designed as bars which are rectangularin cross section and are arranged along the entire fuel cell stack 4. Itis equally possible for the spacers to be formed as smaller cuboidswhich in each case only space apart two or a small number of fuel cells2. The spacers 24 are very stable and simple to produce and assemble ifthey are made from a part of the seal of a fuel cell 2. The spacers thenform an integral part of the seal and therefore of the fuel cell 2.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. A fuel cell module, comprising: a plurality of fuel cells connectedin series form a fuel cell stack; and a stabilizing casing surroundingthe fuel cell stack in its longitudinal direction, the stabilizingcasing including a plurality of elements, matched in a positivelylocking manner to an outer contour of the fuel cell stack, wherein eachof the elements extending in the longitudinal direction of the fuel cellstack contacts at least two sides of the fuel cell stack.
 2. The fuelcell module as claimed in claim 1, wherein the plurality of elementsinclude angle bars, adapted to be placed against an outer edge of thefuel cell stack.
 3. The fuel cell module as claimed in claim 1, whereinthe plurality of elements include at least one U-shaped metal sheet,extended in the longitudinal direction of the fuel cell stack andengaging around the fuel cell stack in cross section on one of its outersides.
 4. The fuel cell module as claimed in claim 1, wherein eachelement is connected to the elements which adjoin it by way of aplurality of connecting elements.
 5. The fuel cell module as claimed inclaim 4, wherein the connecting elements are designed as at least one oftensioning struts and tensioning straps.
 6. The fuel cell module asclaimed in claim 1, wherein each element is provided with a plurality ofinsulating elements on its side which faces the fuel cell stack.
 7. Thefuel cell module as claimed in claim 1, wherein the fuel cells which lieon the outer sides, as seen in the longitudinal direction of the fuelcell stack, are each connected to a connection plate.
 8. The fuel cellmodule as claimed in claim 7, wherein a plurality of tie rods arearranged on each connection plate.
 9. The fuel cell module as claimed inclaim 8, wherein the connection plates are clamped together in thelongitudinal direction of the fuel cell stack, via tensioning devicesadapted to engage on the tie rods.
 10. The fuel cell module as claimedin claim 1, comprising at least two fuel cell stacks, arranged adjacentto but offset from one another perpendicular to the stack direction,inside the stabilizing casing.
 11. The fuel cell module as claimed inclaim 10, comprising an electrically insulating spacer arranged betweenthe two stacks.
 12. The fuel cell module as claimed in claim 11, whereina seal of one of the fuel cells is used as the spacer.
 13. The fuel cellmodule as claimed in claim 2, wherein the plurality of elements includeat least one U-shaped metal sheet, extended in the longitudinaldirection of the fuel cell stack and engaging around the fuel cell stackin cross section on one of its outer sides.
 14. The fuel cell module asclaimed in claim 2, wherein each element is connected to the elementswhich adjoin it by way of a plurality of connecting elements.
 15. Thefuel cell module as claimed in claim 3, wherein each element isconnected to the elements which adjoin it by way of a plurality ofconnecting elements.
 16. The fuel cell module as claimed in claim 2,wherein each element is provided with a plurality of insulating elementson its side which faces the fuel cell stack.
 17. The fuel cell module asclaimed in claim 3, wherein each element is provided with a plurality ofinsulating elements on its side which faces the fuel cell stack.
 18. Thefuel cell module as claimed in claim 4, wherein each element is providedwith a plurality of insulating elements on its side which faces the fuelcell stack.
 19. The fuel cell module as claimed in claim 5, wherein eachelement is provided with a plurality of insulating elements on its sidewhich faces the fuel cell stack.
 20. The fuel cell module as claimed inclaim 1, comprising at least two fuel cell stacks, arranged adjacent tobut offset from one another perpendicular to the stack direction, insidethe stabilizing casing.
 21. The fuel cell module as claimed in claim 20,comprising an electrically insulating spacer arranged between the twostacks.
 22. The fuel cell module as claimed in claim 21, wherein a sealof one of the fuel cells is used as the spacer.
 23. The fuel cell moduleas claimed in claim 2, comprising at least two fuel cell stacks,arranged adjacent to but offset from one another perpendicular to thestack direction, inside the stabilizing casing.
 24. The fuel cell moduleas claimed in claim 23, comprising an electrically insulating spacerarranged between the two stacks.
 25. The fuel cell module as claimed inclaim 24, wherein a seal of one of the fuel cells is used as the spacer.26. A fuel cell module, comprising: a plurality of fuel cells connectedin series form a fuel cell stack; and a stabilizing casing, surroundingthe fuel cell stack at least in a middle area as seen in a longitudinaldirection of the fuel stack, the stabilizing casing including aplurality of elements, matched in a positively locking manner to anouter contour of the fuel cell stack, wherein each of the elementsextending in the longitudinal direction of the fuel cell stack contactsat least two sides of the fuel cell stack.
 27. The fuel cell module asclaimed in claim 26, wherein the plurality of elements include anglebars, adapted to be placed against an outer edge of the fuel cell stack.28. The fuel cell module as claimed in claim 26, comprising at least twofuel cell stacks, arranged adjacent to but offset from one anotherperpendicular to the stack direction, inside the stabilizing casing. 29.The fuel cell module as claimed in claim 28, comprising an electricallyinsulating spacer arranged between the two stacks.
 30. The fuel cellmodule as claimed in claim 29, wherein a seal of one of the fuel cellsis used as the spacer.